This issue is most often caused by third party applications interferring with Steam.
Please uninstall any applications listed in our Programs Which May Interfere with Steam FAQ before following the instructions provided below.
Please exit Steam and browse to your Steam installation folder(see below for default locations for a Steam installation - if you set a different installation directory, you will need to browse to it).
Delete all of the files in this folder except:
Restart your computer.
Then, launch Steam.exe from within the Steam installation folder, and not from a pre-existing shortcut.
With Steam running, re-test the original issue.
If the issue persists, make sure your network is optimized for Steam: Troubleshooting Network Connectivity. Please follow all suggestions in this article, even if your network appears to be functioning correctly.